1. Field of the Invention
This invention pertains to a method conserving power in a mobile communications device.
2. Description of the Related Art
Mobile communications devices have become ubiquitous in modern society. They include, without limitation, cellular telephones, personal digital assistants and pagers. These devices come in a myriad of shapes and sizes as well as different functional capabilities. As the name implies, mobile communications devices are designed to help people remain in contact while on the move. While the following discussion is couched in terms of mobile phones, it is equally applicable to other mobile communications devices.
Presently, mobile phones register with a nearby base station for connection to the mobile network. As part of this registration process, the phone is assigned a paging channel and xe2x80x9ccampsxe2x80x9d thereon, in effect remaining tuned to that particular channel until something causes it to change channels. The paging channel typically sends a page message that contains a temporary mobile station identity (TMSI) to alert a particular mobile phone that a call has been received. The TMSI was previously assigned to the mobile phone by the associated base station with which the phone is presently registered. When the mobile phone detects a match of the TMSI sent by the base station to the TMSI stored in its memory, it will alert the user that an incoming call is to be received. In order to detect a match of the TMSI, the phone must receive and demodulate the paging channel. If the user decides to answer the call (typically by pressing the xe2x80x9cSENDxe2x80x9d or xe2x80x9cTALKxe2x80x9d key), the mobile phone sends further messages to the base station indicating that it is answering the call.
Thus, the phone receives instructions about incoming calls and sends outgoing requests on this paging channel. Because the phone is mobile, the phone needs to know what are alternative channels that it can use as it moves around. These alternative paging channels may or may not be provided by the same base station, but in all likelihood are provided by neighboring base stations. To alert the phone of possible alternative channels, the base station, on whose channel the phone is presently camped, sends out a list of the most likely alternative channels called the Neighbor List, which includes the present channel that the phone is camping on. The phone periodically checks the signal strengths of these alternative channels and determines whether or not to switch paging channels to provide better phone to base station communication.
Because the paging channel is designed to be present at pre-scheduled times on a designated channel and time slot, the phone can be put to xe2x80x9csleep.xe2x80x9d Putting a phone to sleep involves minimizing the computational functions of the phone in order to conserve power. Power conservation is important for battery driven mobile phones. The less power consumed, the longer the phone can operate without interruption. As a result, many efforts are made to conserve power within mobile phones. A prime time to xe2x80x9csleepxe2x80x9d a phone occurs during the time during which the phone does not have to receive and demodulate the paging channel. The phone periodically wakes up, receives and demodulates the paging channel. If the phone detects no page, i.e. there is not a match between the TMSIs, then the phone returns to sleep until time to check the paging channel again.
In typical phones, the phone also uses this awake period to check the neighbor list to determine if reselection is required, or to update the status of which of the neighbor list channels are desirable candidates for reselection. To this end, after the phone receives and demodulates the paging channel, the phone receives signals from the base stations included on its presently existing neighbor list. Based on ANSI-136, the phone knows what sort of signal it expects to receive on each channel of the neighbor list. The received signals are used to compute an Received Signal Strength Indicator (RSSI) for each channel, i.e., each neighboring channel has its own RSSI. Typically an individual running average of the RSSI is kept for each of the available channels on the Neighbor List from the neighboring base stations.
In the process of waking up, a local oscillator must be powered up and allowed to settle at its operating frequency prior to receiving and demodulating the channel on which the phone is presently camped. This delay is due to the fact that to receive and demodulate the paging channel, the phone must be close (within 200 Hz or so) in frequency to the paging channel. It takes time for the oscillator to achieve that degree of accuracy. Presently, during this power up phase, the local oscillator is not used, although it is draining power from the battery, thus shortening the battery life without tangible benefit.
Switching the order in which the mobile phone does its administrative functions during the waking periods may more efficiently use the wasted energy associated with the prior art technique. Specifically, the mobile phone checks the received signal strengths of the channels on the neighbor list while the local oscillator is powering up and may not yet have settled at its operative frequency. By the time the neighbor list has been checked, the local oscillator will have settled at its operative frequency and the mobile phone can check for any paging calls on the paging channel and demodulate the same. This eliminates time wasted waiting for the local oscillator to settle and allows the local oscillator to be turned on for a shorter period of time, thus saving power. Because the local oscillator has not settled, measurements of the channels on the neighbor list may be offset by a predetermined amount to provide a more accurate measurement of the received signal strength. Offsets are determined during a factory calibration routine. These offsets or compensation factors can be stored in a look up table or the like.
Alternate embodiments allow for refinements on this technique. Where one channel clearly exceeds all other measured channels by a factor greater than the values contained in the look up table, the phone does not necessarily need to apply the compensation factors unless needed for the RSSI calculations. Additionally, the relative strengths of the channels on the neighbor list are stored in memory between awake periods and the mobile phone preferably checks the list in ascending order of relative power to minimize interference between channels.